@qingyuan.ni,
Regarding WORDSNET-16060, it is just to update you that starting from the 18.1 versions of Aspose.Words for .NET and Aspose.Words for Java APIs, we had added a feature that you can use to get the StoreItemID property of StructuredDocumentTag. We have made public the StoreItemId property in XmlMapping class. You can use this property to specify the custom XML data identifier for the custom XML data part which shall be used to evaluate the XPath expression. Here is how you can use it:
var doc = new Document("file.docx");
var sdt = (StructuredDocumentTag)doc.GetChild(NodeType.StructuredDocumentTag, 0, true);
Console.WriteLine("Id = {0}", sdt.XmlMapping.StoreItemId);
For more information, please refer to DataBinding Class.